Deck Joist Spacing and Board Gaps
Most wood deck boards run across joists spaced 16 inches on center. Many composite boards — and any boards laid diagonally — require 12 inches on center. Always follow the decking manufacturer's installation guide; it is usually a condition of the warranty.
Joist spacing at a glance
| Decking | Straight layout | Diagonal layout |
|---|---|---|
| 5/4 wood (1 in thick) | 16 in | 12 in |
| 2× wood (1½ in thick) | 16–24 in | 16 in |
| Composite / PVC | 12–16 in (check brand) | 12 in or less |
Gaps between boards
Kiln-dried wood and most composites are typically gapped about 1/8 to 1/4 inch so water drains and boards can move. Wet pressure-treated lumber shrinks as it dries, so builders often lay it tight and let gaps open naturally. Composite boards also need end gaps that depend on temperature — the manufacturer lists them.
Fasteners
Face-screwing uses two screws at every joist. With 16-inch spacing that's 1.5 joist crossings per foot of board, or about 3 screws per linear foot. Hidden clips replace screws on grooved composite boards.
Joist size and span
Joist size depends on span, species and grade. Building departments publish span tables (often based on the American Wood Council's DCA 6 guide) — use them, especially for raised decks.
Count boards, joists and screws with the deck board calculator.